Techniques for coupling in semiconductor devices

ABSTRACT

Techniques for exchange coupling of magnetic layers in semiconductor devices are provided. In one aspect, a semiconductor device is provided. The device comprises at least two magnetic layers, and a spacer layer formed between the magnetic layers, the spacer layer being configured to provide ferromagnetic exchange coupling between the layers, the magnetic layers experiencing anti-ferromagnetic dipole coupling, such that a net coupling of the magnetic layers is anti-ferromagnetic in a zero applied magnetic field. The semiconductor device may comprise magnetic random access memory (MRAM). In another aspect, a method for coupling magnetic layers in a semiconductor device comprising at least two magnetic layers and a spacer layer therebetween, the method comprises the following step. Ferromagnetic exchange coupling is provided of the magnetic layers, the magnetic layers experiencing anti-ferromagnetic dipole coupling, such that a net coupling of the magnetic layers is anti-ferromagnetic in a zero applied magnetic field.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices and, moreparticularly, to coupling of magnetic layers in semiconductor devices.

BACKGROUND OF THE INVENTION

Semiconductor devices, such as magnetic random access memory (MRAM)devices, use magnetic bits to store information. A free layer of thedevice serves as the magnetic bit. The information is stored as thedirection of magnetization of the bit, either pointing right or left, tostore “1” or “0.” When the bit is sitting in a zero applied magneticfield, its magnetization is stable, pointing either right or left. Theapplication of a magnetic field can be used to write information to thebit by switching the magnetization of the bit from right to left, orvice versa. One of the important requirements for data storage is thatthe magnetization of the bit not change direction when there is a zeroapplied field, or only a small applied field.

Unfortunately, in practice, the magnetization of the bits does changedirections unintentionally, due to thermal activation. Thermalactivation occurs when thermal energy from the environment surroundingthe bit overcomes an activation energy barrier to change the directionof magnetization. The occurrences of thermal activation should beminimized. The resulting error rate due to thermally activated switchingis called the soft error rate (SER).

One of the objectives in designing MRAM devices is to have low operatingpower and small area. Low operating power and small area requires a lowswitching field for the bit. A low switching field uses a low switchingcurrent, which in turn uses less power. Further, lower switchingcurrents require smaller switches, which occupy less space.

As the area of the bits becomes increasingly smaller, a process referredto as “scaling” due to the fact that the bit area is scaled down toallow for more bits in the same area, the SER becomes worse. Asmentioned above, the activation energy barrier may be overcome due tothermal energy, resulting in thermal activation. Therefore, it isdesirable to have a large enough activation energy barrier to preventthermal activation and the magnetization of the bit changing direction.

According to single domain theory, the activation energy barrier of thebit is proportional to the volume of the bit. Therefore, as the area isscaled down, and if nothing else changes, the activation energy barrierdecreases and the SER becomes unacceptably large. A conventional, simplesolution to this problem would be to increase the thickness of the bitas the area of the bit is scaled down, to maintain a large enough volumeto keep the energy activation barrier large enough. However, thistechnique quickly runs into problems because a greater magnetic field isrequired to switch the magnetization of a thicker bit. Thus, a primarygoal of the scaling process becomes to make the area of the bit smaller,but to maintain the activation energy barrier and the switching field,i.e., preventing the activation energy barrier from becoming too smalland preventing the switching field from becoming too large.

U.S. Pat. No. 6,545,906, issued to Savtchenko et al. (hereinafter“Savtchenko”), discloses a new type of free layer for use in MRAMdevices. The free layer is composed of two magnetic layers separated bya non-magnetic spacer layer. In a zero applied magnetic field, the twomagnetic layers have moments that are lined up anti-parallel to eachother due to anti-ferromagnetic dipole coupling and exchange coupling.

The spacer layer may provide some exchange coupling. This exchangecoupling, as described in Savtchenko, is however limited toanti-ferromagnetic exchange coupling.

It would be desirable to be able to produce semiconductor devices thatallow for a reduction in area, yet maintain an activation energy barrierand a switching field, such that occurrences of thermal activation areminimized.

SUMMARY OF THE INVENTION

The present invention provides techniques for exchange coupling ofmagnetic layers in semiconductor devices. In one aspect of theinvention, a semiconductor device is provided. The device comprises atleast two magnetic layers, and a spacer layer formed between themagnetic layers, the spacer layer being configured to provideferromagnetic exchange coupling between the layers, the magnetic layersexperiencing anti-ferromagnetic dipole coupling, such that a netcoupling of the magnetic layers is anti-ferromagnetic in a zero appliedmagnetic field. The semiconductor device may comprise magnetic randomaccess memory (MRAM).

In another aspect of the invention, a method for coupling magneticlayers in a semiconductor device comprising at least two magnetic layersand a spacer layer therebetween comprises the following step.Ferromagnetic exchange coupling is provided of the magnetic layers, themagnetic layers experiencing anti-ferromagnetic dipole coupling, suchthat a net coupling of the magnetic layers is anti-ferromagnetic in azero applied magnetic field.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary semiconductor deviceaccording to an embodiment of the present invention;

FIG. 2 is a phase diagram for two identical coupled layers according toan embodiment of the present invention;

FIG. 3 is a hysteresis loop for a sample in the spin-flop phase, aspredicted by the single domain model according to an embodiment of thepresent invention;

FIG. 4 is a graph illustrating the choosing of parameters for anexemplary circular semiconductor device to attain desiredcharacteristics according to an embodiment of the present invention; and

FIG. 5 is a graph illustrating standard measurements of exchangecoupling as a function of spacer layer thickness.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating an exemplary semiconductor device 100.Semiconductor device 100 comprises magnetic layer 102, spacer layer 104and magnetic layer 106. Magnetic layers 102 and 106, as shown in FIG. 1,have an elliptical shape. However, in accordance with the teachingspresented herein, each of magnetic layers 102 and 106 may have anysuitable non-elliptical shape, such as a circular shape.

Each of magnetic layers 102 and 106 may comprise an element including,but not limited to, nickel, cobalt, iron, manganese and combinationscomprising at least one of the foregoing elements. In an exemplaryembodiment, magnetic layer 102 and/or magnetic layer 106 comprisesNi₈₀Fe₂₀. The composition of magnetic layer 102 may be the same as thecomposition of magnetic layer 106. Alternatively, the composition ofmagnetic layer 102 may be different from the composition of magneticlayer 106.

The thickness of magnetic layer 102 may be the same as the thickness ofmagnetic layer 106. Alternatively, the thickness of magnetic layer 102may be different from the thickness of magnetic layer 106. In anexemplary embodiment, the thickness imbalance between magnetic layer 102and magnetic layer 106 is less than or equal to about ten percent. Forexample, the thickness imbalance between magnetic layer 102 and magneticlayer 106 may be less than or equal to about five percent.

Each of magnetic layers 102 and 106 has an intrinsic anisotropy. In anexemplary embodiment, magnetic layers 102 and 106 have the sameintrinsic anisotropy.

Spacer layer 104 may comprise a transition metal. Suitable transitionmetals include, but are not limited to, chromium, copper, ruthenium,rhodium, palladium, rhenium, osmium, iridium, platinum and combinationscomprising at least one of the foregoing transition metals. In anexemplary embodiment, spacer layer 104 comprises ruthenium. In a furtherexemplary embodiment, spacer layer 104 is non-magnetic.

Spacer layer 104 may comprise an insulating layer. Suitable insulatinglayers include, but are not limited to, layers comprising aluminumoxide.

In an exemplary embodiment, spacer layer 104 has a thickness of greaterthan or equal to about 0.5 nanometers (nm). For example, spacer layer104 may have a thickness of from about one nm to about 1.6 nm. Inanother exemplary embodiment, spacer layer 104 has a thickness ofgreater than or equal to about two nm. For example, spacer layer 104 mayhave a thickness of from about two nm to about 2.8 nm.

Spacer layer 104 may comprise an incomplete layer. For example, spacerlayer 104 may comprise pinholes.

In another embodiment, spacer layer 104 may comprise a weakferromagnetic layer. Such a layer comprises an alloy with a compositioncomprising at least one of magnesium, iron, cobalt and nickel.

Spacer layer 104 provides exchange coupling of magnetic layers 102 and106. The exchange coupling of magnetic layers 102 and 106 by spacerlayer 104 may be either ferromagnetic or anti-ferromagnetic, due in partto the characteristics of spacer layer 104, including, but not limitedto, the composition of spacer layer 104 and/or the thickness of spacerlayer 104. In an exemplary embodiment, the exchange coupling of magneticlayers 102 and 106 by spacer layer 104 is ferromagnetic. Havingferromagnetic exchange coupling as disclosed herein is unique, given thefact that, as will be described in detail below, a net coupling ofmagnetic layers 102 and 106 is anti-ferromagnetic. Further, since thethickness of spacer layer 104 impacts the exchange coupling of magneticlayer 102 and 106, in another exemplary embodiment, the thickness ofspacer layer 104 is varied to attain ferromagnetic exchange coupling ofmagnetic layers 102 and 106.

Magnetic layers 102 and 106, in a zero applied magnetic field, will havemagnetic moments that line up anti-parallel to each other due toanti-ferromagnetic dipole coupling. Anti-ferromagnetic dipole couplingoccurs as a result of the termination of the magnetic materials at theends of the bit and the magnetic poles that are formed there. Accordingto the teachings herein, the net coupling, i.e., the sum of the exchangecoupling and the dipole coupling, of magnetic layers 102 and 106 isanti-ferromagnetic when semiconductor device 100 is in the presence of azero applied magnetic field. Techniques for attaining ferromagneticexchange coupling when the net coupling of magnetic layers 102 and 106is anti-ferromagnetic, in the presence of a zero applied magnetic field,will be described in detail below.

The techniques, as provided herein, derive the exact single domaintheory that describes the phenomena wherein the exchange coupling ofmagnetic layers 102 and 106 is ferromagnetic, but the net coupling isanti-ferromagnetic. This new theoretical understanding makes it clearthat ferromagnetic exchange coupling is beneficial. For ease ofreference, the following description will be divided into the followingsections: (I) Single Domain Model and (II) Identifying FerromagneticCoupling.

I. Single Domain Model

For simplicity, it is assumed that magnetic layers 102 and 106 have thesame thickness t. The results, however, are not substantially affectedby small thickness imbalances. Thickness imbalance tolerances aredescribed in detail above. The single domain calculation also assumesthat magnetic layers 102 and 106 are in the shape of an ellipse, havethe same intrinsic anisotropy H_(i) (in the direction of the long axisof the ellipse), have magnetization M_(s), have width b, have length aand are coupled together by an exchange coupling J. However, as washighlighted above for example, magnetic layers 102 and 106 may have anysuitable shape. An exchange coupling wherein J is greater than zerocomprises ferromagnetic coupling and an exchange coupling wherein J isless than zero comprises antiferromagnetic coupling.

FIG. 2 is a phase diagram for two identical coupled layers. Namely, thephase diagram in FIG. 2 classifies all of the possible behaviors thatcan be found when a magnetic field is swept along the easy axis ofsemiconductor device 100, as referred to above in conjunction with thedescription of FIG. 1. The phases are organized according to the type ofhysteresis loop that the single domain model predicts. The phase on theleft of the diagram, labeled “spin-flop,” is the relevant phase formagnetic random access memory (MRAM). The phase diagram in FIG. 2 makesit clear that, even though most of the phase exists where J is negative(antiferromagnetic coupling), this spin-flop phase extends into theregion where J is positive (ferromagnetic coupling).

FIG. 3 is a hysteresis loop for a sample in the spin-flop phase, aspredicted by the single domain model. The spin-flop field H_(sf) is thefield at which the MRAM is written, and the saturation field H_(xsat) isthe field wherein the magnetic moments of magnetic layers 102 and 106line up parallel to each other.

H_(xsat) determines the write margins of semiconductor device 100, asreferred to above in conjunction with the description of FIG. 1. In atoggle mode, staying below H_(xsat) during full select is needed. Indirect mode, staying below the related saturation field (along the halfselect direction) during half select is needed. Toggle and direct writemodes are described in detail in Savtchenko, the disclosure of which isincorporated by reference herein. Therefore, it is beneficial to be ableto control parameters, such as the two fields H_(sf) and H_(xsat).

A third parameter that needs to be controlled is that of the activationenergy in zero field E_(a). Solving the single domain model for thethree parameters, H_(sf), H_(xsat) and E_(a) gives the following values,$\begin{matrix}{H_{xsat} = {{8\quad\pi\quad M_{s}n_{x}\frac{t}{b}} - \frac{2J}{M_{s}t} - H_{i}}} & (1) \\{H_{sf} = \left\lbrack {H_{i}\left( {{8\quad\pi\quad M_{s}n_{y}\frac{t}{b}} - \frac{2J}{M_{s}t} + H_{i}} \right)} \right\rbrack^{1/2}} & (2) \\{{E_{a} = \frac{\pi\quad M_{s}{abtH}_{i}}{4}},} & (3)\end{matrix}$wherein n_(x) and n_(y) are the reduced demagnetizing factors formagnetic layers 102 and 106 having an elliptical shape (for a circle,n_(x) equals n_(y) which equals 0.79, and for an aspect ratio equal totwo, n_(x) equals 0.32 and n_(y) equals 0.90). Equations 1 through 3illustrate that making J positive (ferromagnetic coupling) will reducethe switching fields.

A more detailed analysis shows that for magnetic layers 102 and 106having a circular shape, the parameters t, H_(i) and J can always bechosen to satisfy any choice of H_(sf), H_(xsat) and E_(a),$\begin{matrix}{H_{i} = {\frac{H_{xsat}}{4}\left\lbrack {\left( {1 + {8\frac{H_{sf}^{2}}{H_{xsat}^{2}}}} \right)^{1/2} - 1} \right\rbrack}} & (4) \\{J = {\frac{64(0.78)E_{a}^{2}}{\pi\quad b^{5}H_{f}^{2}} - {\frac{2E_{a}}{\pi\quad b^{2}}\left( {1 + \frac{H_{xsat}}{H_{i}}} \right)}}} & (5) \\{t = {\frac{4\quad E_{a}}{\pi\quad b^{2}M_{s}H_{t}}.}} & (6)\end{matrix}$

Equations 4 through 6 further show how to choose the parameters t, H_(i)and J in order to satisfy any desired activation energy, write field(determined by H_(sf)) and write margin (determined by H_(xsat)).

FIG. 4 is a graph illustrating the choosing of parameters for anexemplary circular semiconductor device to attain desiredcharacteristics. Namely, the parameters t, H_(i) and J are chosen tosatisfy a desired activation energy, write field and write margin. Inthis exemplary embodiment, a free layer is designed with an E_(a) equalto 60 kT (wherein k is Boltzmann's constant and T is absolutetemperature), H_(sf) equal to 50 oersted (Oe) and H_(xsat) equal to 200Oe.

Equation 4 sets H_(i) equal to 11.2 Oe. The parameters t and J are thensimply chosen from Equations 5 and 6 depending on the junction width.For example, a junction width of 200 nm requires a t equal to 5.8 nm,and a J equal to 0.29 ergs per square centimeter (erg/cm²). Note thatthe required J is positive, indicating ferromagnetic coupling. By usingEquations 4 through 6 as the junction width b is reduced, E_(a) can beheld fixed at a large enough value to essentially eliminate the softerror rate (SER), while simultaneously also holding H_(sf) and H_(xsat)fixed.

II. Identifying Ferromagnetic Coupling

Identifying when a structure utilizes ferromagnetic exchange coupling isstraightforward. J depends on the composition of the spacer layer, thecomposition of the magnetic layers and the spacer layer thickness.Standard measurements, such as those shown in FIG. 5, are reported inthe literature. See, for example, S. S. P. Parkin, GiantMagnetoresistance and Oscillatory Interlayer Coupling in PolycrystallineTransition Metal Multilayers, in U LTRATHIN MAGNETIC STRUCTURES II (B.Heinrich et al., eds., 1994), the disclosure of which is incorporated byreference herein. Namely, FIG. 5 is a graph illustrating standardmeasurements of exchange coupling as a function of spacer layerthickness. In FIG. 5, J is measured for two magnetic layers comprisingNi₈₀CO₂₀, sandwiched around an exchange spacer layer comprisingruthenium, for various different spacer layer thicknesses.

The data show that, for these materials, the coupling is ferromagneticfor a spacer layer having a thickness of between about one nm to about1.6 nm, and also between about 2.2 nm to about 2.8 nm. Therefore, oneway to determine if a given structure exhibits ferromagnetic exchangecoupling would be to measure the composition of the spacer layer, thecompositions of the magnetic layers and the thickness of the spacerlayer, and then reference the corresponding data in the literature,i.e., in FIG. 5.

Another way to determine if a given structure exhibits ferromagneticexchange coupling is to measure the fields H_(sf) and H_(xsat). Then,make a comparison structure that uses a different spacer layer materialwhich is know to give zero exchange coupling. Spacer layer materialsthat give zero exchange coupling are known, and include, aluminum,titanium, zirconium, hafnium and combinations comprising at least one ofthe foregoing materials. The comparison structure may also employ athick spacer material, i.e., one that always gives zero exchangecoupling (having a thickness of greater than or equal to about sevennm). The fields H_(sf) and H_(xsat) of this comparison structure maythen be measured. If the fields H_(sf) and H_(xsat) in the comparisonstructure are larger than the given structure, then from Equations 1through 3 the original structure must utilize a positive J(ferromagnetic exchange coupling).

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope or spirit of the invention.

1. A semiconductor device comprising: at least two magnetic layers; anda spacer layer formed between the magnetic layers, the spacer layerbeing configured to provide ferromagnetic exchange coupling between thelayers, the magnetic layers experiencing anti-ferromagnetic dipolecoupling, such that a net coupling of the magnetic layers isanti-ferromagnetic in a zero applied magnetic field.
 2. The device ofclaim 1, comprising magnetic random access memory (MRAM).
 3. The deviceof claim 1, wherein the net coupling of the magnetic layers comprises asum of the exchange coupling and the dipole coupling.
 4. The device ofclaim 1, wherein each of the magnetic layers has a same thickness. 5.The device of claim 1, wherein a difference in a thickness of each ofthe magnetic layers relative to one another is less than or equal toabout ten percent.
 6. The device of claim 1, wherein the magnetic layersare elliptical.
 7. The device of claim 1, wherein the magnetic layersare circular.
 8. The device of claim 1, wherein each of the magneticlayers has a same intrinsic anisotropy.
 9. The device of claim 1,wherein one or more of the magnetic layers comprises an element selectedfrom the group consisting of nickel, cobalt, iron, manganese andcombinations comprising at least one of the foregoing elements.
 10. Thedevice of claim 1, wherein one or more of the magnetic layers compriseNi₈₀Fe₂₀.
 11. The device of claim 1, wherein the spacer layer comprisesa transition metal.
 12. The device of claim 11, wherein the transitionmetal is selected from the group consisting of chromium, copper,ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum andcombinations comprising at least one of the foregoing transition metals.13. The device of claim 1, wherein the spacer layer comprises anincomplete layer.
 14. The device of claim 1, wherein the spacer layercomprises pinholes.
 15. The device of claim 1, wherein the spacer layercomprises an insulator.
 16. The device of claim 1, wherein the spacerlayer comprises aluminum oxide.
 17. The device of claim 1, wherein thespacer layer is non-magnetic.
 18. The device of claim 1, wherein thespacer layer comprises a weak ferromagnet.
 19. The device of claim 18,wherein the weak ferromagnet comprises magnesium, iron, cobalt, nickeland combinations comprising at least one of the foregoing elements. 20.The device of claim 1, wherein the spacer layer has a thickness of fromabout one nanometer to about 1.6 nanometers.
 21. The device of claim 1,wherein the spacer layer has a thickness of from about two nanometers toabout 2.8 nanometers.
 22. A method for coupling magnetic layers in asemiconductor device comprising at least two magnetic layers and aspacer layer therebetween, the method comprising the step of providingferromagnetic exchange coupling of the magnetic layers, the magneticlayers experiencing anti-ferromagnetic dipole coupling, such that a netcoupling of the magnetic layers is anti-ferromagnetic in a zero appliedmagnetic field.
 23. The method of claim 22, wherein the spacer layercomprises a transition metal.
 24. The method of claim 23, wherein thetransition metal is selected from the group consisting of chromium,copper, ruthenium, rhodium, palladium, rhenium, osmium, iridium,platinum and combinations comprising at least one of the foregoingtransition metals.
 25. The method of claim 22, further comprising thestep of varying a thickness of the spacer layer to attain ferromagneticexchange coupling of the magnetic layers.